Optical fibers are used to communicate information optically. Typically, an optical transmitter at one end of the fiber will transmit an optical signal into one end of the fiber, and an optical receiver at the other end of the fiber will receive the optical signal after having been communicated over the length of the optical fiber.
An optical fiber is composed of primarily two concentric layers. The inner-most layer is called a “core”, which conducts the light through the length of the fiber. The core is surrounded by a layer called a “cladding” which provides a material that is of a different index of refraction than the core, which allows the light to propagate through the core by principles of total internal reflection. A protective layer called a “coating” may perhaps be placed outside of the cladding layer to provide protection to the cladding and core.
There are many types of conventional optical fibers available. Optical fibers may be characterized by length, core diameter, mode type (single-mode versus multi-mode), index type for multi-mode fibers (step-index verses graded-index), dispersion shifted characteristic (e.g., Dispersion Shifted (DS) and Non-Zero Dispersion Shifted (NZDS)), core and cladding layer materials (glass versus plastic), and so forth. Each type of fiber product has particular characteristics, and have a particular application or range of applications that it may be applied to.
Even in ideal circumstances, each of these fiber types has particular characteristics that have the effect of degrading the optical signal. For instance, the optical signal may have certain dispersion characteristics. Dispersion is essentially the tendency of the optical signal to spread out as the signal passes through the optical fiber. The optical fiber also has a certain attenuation per unit length that varies according to optical frequency. There are a number of other known effects that have an impact upon optical signal degradation.
In addition, a number of events might happen to the optical fiber, either during the course of its manufacture, or in the field, that might affect the degradation of the optical signal as it passes through the optical fiber. For instance, if the optical fiber has difference indices of refraction in the core or cladding, or dopant concentration deviations, or microbends in the core-cladding interface, the transfer function that the optical signal may be subjected to in the fiber may be different than that designed.
There are a wide-variety of possible optical fibers, a wide variety of possible manufacturing defects in the optical fiber, and a wide variety of damage caused to the optical fiber in the field. There are at least two known solutions for testing whether an optical transceiver would operate with a particular set of optical fiber defects.
One conventional method is to simulate the transfer function of the defective optical fiber. This can be done by inputting the optical signal to an expensive signal manipulation component that imposes the transfer function on the optical signal, and then provides a resultant optical signal to an optical receiver.
Another conventional method is to actually find an optical cable that includes an optical fiber that happens to have the same conditions that are to be tested for. Since there are many possible defects that are desired to test for, it can be quite difficult to search for and find an actual cable the mirrors the defects to be tested for. In some cases, such a cable may not even exist, despite an extensive search. In addition, even if such a cable was found, the cable might have unstable performance. For instance, a lengthy stretch of the defective optical fiber might have a different transfer function when the coil is laid on one side as compared to the other side.